The present invention relates to a developing apparatus and process for the liquid development of electrostatic charge images on an image carrier material. The apparatus includes a device for spraying a liquid developer onto the image carrier material to be developed and a developing electrode connected to a direct-current voltage source.
An apparatus of this type for the development of an e1ectrostatic charge image electrophotographic copying material is described in German Pat. No. 2,507,221 corresponding to U.S. Pat. No. 4,073,266. This apparatus is equipped with a pair of feeding rollers for applying to a copier material a toner dispersion, as well as with a distributor roller for evenly distributing the toner dispersion on the charge image. The spray pipe for spraying the toner dispersion is fitted above the distributor roller and has series of nozzles in its pipe jacket arranged symmetrically to the vertical center line of the spray pipe. The voltage applied to the distributor roller is at least as great as the charge level at the non-image area of the charge image and no greater than the charge level at the image points of a full-tone area of the charge image. The voltage applied to the distributor roller is preferably in the range between about 20 and 350 volts. The distributor roller operates as a back electrode or developing electrode and the toner deposited on the surface of the distributor roller is rinsed off by spraying with the toner dispersion. The toner runs into a collecting tray to be reused.
German Offenlegungsschrift No. 2,138,254 describes an electrophotographic reverse developing apparatus having one or more developing electrode rolls carrying a bias voltage of the same polarity as the charge image to be developed and equipped with devices for supplying toner dispersion to the developing electrode rolls. The developing electrode rolls are not in direct contact with the copying material and the toner particles of the toner dispersion have the same polarity as the charge image. The liquid developer is applied directly to the developing electrode rolls and then is passed from the developing electrode rolls to the recording carrier. Arranged underneath the recording carrier are conductive rolls, which support the underside of the recording carrier. The conductive rolls are cleaned by means of flexible brushes.
German Offenlegungsschrift No. 2,332,253 discloses a developing apparatus for an electrophotographic copying material which passes, in a developing zone, under applicator rollers which are wetted from above with a toner dispersion. In this way, a film of the toner dispersion is applied to the copying material. At the end of the developing zone, there is a pair of squeeze rollers which, by squeezing the copying material, remove excessive toner dispersion from the surface of the copying material.
German Auslegeschrift No. 2,149,889 discloses a developing apparatus which has a roller or a continuous circulating web as an applicator element for applying the liquid toner to a recording material bearing charge images. At a point on the path of movement of the applicator element between emergence from the developer bath and application to the recording material, there is a device for reducing the amount of liquid carried along by the toner deposited on the applicator element. This device can be a corona discharge electrode. Excess toner, or toner left behind on the applicator element, is removed after development by means of a wiper blade, a rotating brush or other devices.
A disadvantage common to the known developing apparatus is that the toner supply is not applied evenly under all conditions. In particular, the reproduction of full tone areas, which must be covered homogeneously with toner, the background-free reproduction of non-image areas and, especially, the reproduction of screen images faithful to tonal values on the recording materials leaves something to be desired.
In the case of many photoconductor layers used in practice, complete discharge by exposure is difficult. In practice, charges still remain even at non-image points exposed to the light. The potential remaining on the surfaces of photoconductor layers following exposure can be 50 to 100 volts and more. This behavior of the photoconductor layers is attributable to the strong dependency of the motion of charge carriers generated by light on the prevailing field strength.
As already mentioned in conjunction with the state of the art, briefly described above, in order to reproduce solid areas homogeneously covered with toner, so-called back electrodes or developing electrodes in the form of rolls, rollers or plates are utilized, the shapes of which are adapted to the particular recording carrier to be developed. However, when using such electrodes, the residual charges remaining at non-image areas are included in developing so that the developed images are not background free. It is therefore necessary to apply a counter voltage to the developing electrodes to compensate for the charges on the photoconductor layer. It is only by this means that the development of fully background-free images is made possible. However, the application of a voltage to the developing electrode causes the deposition of toner on the electrode, reducing its efficiency very rapidly. Since, however, it is essential that developing conditions remain constant specifically in the case of screen reproductions faithful to tonal values, this means that the developing electrode must constantly be cleaned, simple general rinsing not being enough. Additional mechanical or electrical forces must be used to remove the toner adhering to the developing electrode. This cleaning process requires a great deal of equipment and time. Disposal of the toner washed off or mechanically removed also requires considerable effort.